This invention relates to a magnetron and more particularly to a structure of a magnetron which enables it effectively to attenuate at the inside of the tube envelope a high frequency (hereinafter called as h.f.) noise which may leak through the cathode terminal of the tube.
In a magnetron a part of the h.f. electromagnetic energy can leak toward the input terminal of a cathode through a holder which acts to supply a heating power to the cathode.
This h.f. leak is called h.f. noise and its frequency range extends from the KHz to the GHz range. The strongest energy is of course the fundamental wave corresponding to the resonant frequency of the resonant cavity. The h.f. noise not only interferes with other electronic equipment but also is harmful to a human body when its power is strong. To prevent the h.f. leak, a shield box surrounding the cathode input terminal is provided so as to prevent the h.f. energy from leaking out. In the shield box, an LC filter which is composed of an inductor and a capacitor is connected to the cathode lead. However, because the conventional LC filter is connected to the resonant cavities of the magnetron through the cathode input section, both the filter and the cavities interfere with each other or the filter is liable to be burned off. Moreover, requirement about attenuation rate of the filter becomes severe. Further it is impossible to separate the filter section from the magnetron body, as they are unified together. Consequently a degree of freedom about installation of the magnetron in a microwave oven is extremely restricted. It is therefore expected that above-mentioned problems will be solved by utilizing a cathode input section functioning as an h.f. filter. On the other hand a magnetron in which an electromagnetic energy absorber such as a ferrite bead is wound around the cathode holder is well known. But there is the possibility that the ferrite bead can overheat because of the absorption of electromagnetic energy and the receiving of the heat from the cathode and consequently generate a large quantity of gas which is detrimental to the operation of the tube.